Patent GB-628712-A (GB)
Title: Improvements in and relating to the braking of electrically propelled vehicles
IPC: B60L7/04
Filed: 1947-10-07
Abstract: 628,712. Control of D.C. motors. ENGLISH ELECTRIC CO., Ltd., and CALDBECK. C. J. Oct. 7, 1947, No. 26897. [Class 38(iii)] In a braking system for an electrically propelled vehicle the armature of the driving motor is connected across the motor starting resistance and a shunt field winding of the motor is excited from the external supply so that the motor acts as a generator exerting a braking torque on the vehicle wheels, the field winding being connected, at least during the latter part of the braking period, to the supply through a part of the motor starting resistance. and means are provided for automatically disconnecting the starting resist-. ance from the supply, even if the brake switch is held in the " on " position, when the speed of the vehicle falls-below a predetermined value. The disconnection of the starting resistance from the supply when the vehicle is at rest reduces the possibility of shock due to defective insulation of the starting resistance. A traction motor 1 has series and shunt field windings 1a and 1b respectively and is connectable to supply-mains through a reversing switch 2, a starting resistance 3, and line contactors 4 and 5. A controller 8 regulates the operation of the line contactors and contactors for the sequential removal of sections of the starting resistance. A diverting. resistance 15 may be connected across the series field winding on the highest step of acceleration when a braking switch 16 is actuated to close contacts 16a and 16b and to open contacts 16c, the line and resistance step contactors are opened and contactors 17 and 18 are closed. The motor armature is connected across a portion 3a-3b of the starting resistance and its shunt field winding is connected across the main supply terminals in series with a resistor 19. A second step of braking operation is provided by closure of contacts 16d, to connect the operating-coil 20a of contactor 20 across the motor armature. Contactors 20 and 11 are closed and contactor 18 opens so that the shunt field winding is now connected across the mains through a part of the starting resistance including the section 3a-3c thereof. Since this section carries the motor armature current the voltage-drop thereacross is substantially proportional to the motor speed, and the excitation of the shunt field winding therefore increases as the speed decreases. When the motor speed. and the voltage applied to the coil 20a falls below a predetermined value the contactors 20 and 11 open, to disconnect the field winding 1b from the point 3c and the starting resistance from supply terminal 6, even if the brake switch is held in its second position. If the switch 16 is held in either of its braking positions when the motor is stationary, the resistor 19 is included in the circuit of the shunt field winding and over-heating is prevented. Interlocks prevent false operation if the braking- switch is operated with the controller 8 in a power position. The system may be adopted to the type of control in which sections of the starting resistance are connected in parallelduring starting operation.